From cppreference.com
std::gslice is the selector class that identifies a subset of
indices defined by a multi-level set of strides and sizes. Objects of type std::gslice can be used as indices with valarray's operator[] to select, for example, columns of a multidimensional array represented as a valarray.
Given the starting value s, a list of strides ij and a list of sizes dj, a std::gslice constructed from these values selects the set of indices kj=s+Σj(ijdj).
For example, a gslice with starting index 3, strides {19,4,1} and lengths {2,4,3} generates the following set of 24=2*4*3 indices:
3+0*19+0*4+0*1=3,3+0*19+0*4+1*1=4,3+0*19+0*4+2*1=5,3+0*19+1*4+0*1=7,3+0*19+1*4+1*1=8,3+0*19+1*4+2*1=9,3+0*19+2*4+0*1=11,...3+1*19+3*4+1*1=35,3+1*19+3*4+2*1=36
It is possible to construct std::gslice objects that select some indices more than once: if the above example used the strides {1,1,1}, the indices would have been {3, 4, 5, 4, 5, 6, ...}. Such gslices may only be used as arguments to the const version of
, otherwise the behavior is undefined.
Member functions
constructs a generic slice
(public member function)returns the parameters of the slice
(public member function)std::gslice::gslice
gslice() (1) gslice(std::size_tstart,conststd::valarray<std::size_t>&sizes,conststd::valarray<std::size_t>&strides); (2) gslice(constgslice&other); (3) Constructs a new generic slice.
1) Default constructor. Equivalent to gslice(0,std::valarray<std::size_t>(),std::valarray<std::size_t>()). This constructor exists only to allow construction of arrays of slices.
2) Constructs a new slice with parameters start, sizes, strides.
3) Constructs a copy of other.
Parameters
start - the position of the first element sizes - an array that defines the number of elements in each dimension strides - an array that defines the number of positions between successive elements in each dimension other - another slice to copy std::slice::start, size, stride
std::size_tstart()const; (1) std::valarray<std::size_t>size()const; (2) std::valarray<std::size_t>stride()const; (3) Returns the parameters passed to the slice on construction - start, sizes and strides respectively.
Parameters
(none)
Return value
The parameters of the slice -- start, sizes and strides respectively.
Complexity
Constant.
Example
Demonstrates the use of gslices to address columns of a 3D array:
Run this code
#include<iostream>#include<valarray>voidtest_print(std::valarray<int>&v,intplanes,introws,intcols){for(intr=0;r<rows;++r){for(intz=0;z<planes;++z){for(intc=0;c<cols;++c)std::cout<<v[z*rows*cols+r*cols+c]<<' ';std::cout<<" ";}std::cout<<'\n';}}intmain(){std::valarray<int>v=// 3d array: 2 x 4 x 3 elements{111,112,113,121,122,123,131,132,133,141,142,143,211,212,213,221,222,223,231,232,233,241,242,243};// int ar3d[2][4][3]std::cout<<"Initial 2x4x3 array:\n";test_print(v,2,4,3);// update every value in the first columns of both planesv[std::gslice(0,{2,4},{4*3,3})]=1;// two level one strides of 12 elements// then four level two strides of 3 elements// subtract the third column from the second column in the 1st planev[std::gslice(1,{1,4},{4*3,3})]-=v[std::gslice(2,{1,4},{4*3,3})];std::cout<<"\n""After column operations:\n";test_print(v,2,4,3);}Output:
Initial 2x4x3 array: 111 112 113 211 212 213 121 122 123 221 222 223 131 132 133 231 232 233 141 142 143 241 242 243 After column operations: 1 -1 113 1 212 213 1 -1 123 1 222 223 1 -1 133 1 232 233 1 -1 143 1 242 243 Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 it was unclear whether a default constructed generic slice is usable it is usable (as an empty subset) See also
get/set valarray element, slice, or mask
(public member function)
BLAS-like slice of a valarray: starting index, length, stride
(class)
proxy to a subset of a valarray after applying a gslice
(class template)